Disorder-induced collapse of the electron-phonon coupling in MgB 2 observed by Raman spectroscopy

The Raman spectrum of the superconductor ${\mathrm{MgB}}_{2}$ has been measured as a function of the ${T}_{c}$ of the film. A striking correlation is observed between the ${T}_{c}$ onset and the frequency of the ${E}_{2g}$ mode. Analysis of the data with the McMillan formula provides clear experimental evidence for the collapse of the electron-phonon coupling at the temperature predicted for the convergence of two superconducting gaps into one observable gap. This gives indirect evidence of the convergence of the two gaps and direct evidence of a transition to an isotropic state at 19 K. The value of the electron-phonon coupling constant is found to be 1.19 for films with ${T}_{c}\ensuremath{\simeq}39\mathrm{K}$ and 0.80 for films with ${T}_{c}l~19\mathrm{K}.$